The present application relates to surface coil antennae for nuclear magnetic resonance imaging and, more particularly, to a novel nuclear magnetic resonance imaging antenna subsystem having a plurality of surface coils disposed in non-orthogonal relationship, and preferably in the same plane.
It is known to use a surface coil as a receiving antenna in an in vivo nuclear magnetic resonance (NMR) experiment; a surface coil is generally more sensitive to smaller volumes than the considerably larger volume coils typically utilized with head and/or body imaging NMR equipment. In the typical NMR experiment, the sample to be analyzed is immersed in a substantially homogeneous static magnetic field B.sub.O, typically directed along one axis, e.g. the Z axis, of a three-dimensional Cartesian set of coordinates. Under the influence of the magnetic field B.sub.O, the nuclei (and therefore the net magnetization M) of atoms having an odd-number of nucleons precess or rotate about the axis of the field. The rate, or frequency, at which the nuclei precese is dependent upon the strength of the applied magnetic field and on the nuclear characteristics. The angular frequency of precession .omega. is defined as the Larmor frequency and is given by the equation: .omega.=.gamma.B.sub.O, in which .gamma. is the gyromagnetic ratio (constant for each type of nucleus). The frequency at which the nuclei precess is therefore substantially dependent on the strength of the magnetic field B.sub.O, and increases with increasing field strength. Because the precessing nucleus is capable of absorbing and re-radiating electromagnetic energy, a radio-frequency (RF) magnetic field at the Larmor frequency can be utilized to excite the nuclei and receive imaging response signals therefrom. It is possible, by superimposing one or more magnetic field gradients of sufficient strength, to spread out the NMR signal spectrum of the sample and thereby distinguish NMR signals arising from different spatial positions in the sample, based on their respective resonant frequencies. Spatial positions of the NMR signals are determinable by Fourier analysis and knowledge of the configuration of the applied magnetic field gradient, while chemical-shift information can be obtained to provide spectroscopic images of the distribution of a particular specie of nucleus within the imaged sample.
For NMR imaging at relatively high static field B.sub.O magnitudes (typically in excess of 0.5 Tesla (T)), having associated Larmor frequencies greater than about 10 MHz., surface coils utilized as imaging or spectroscopy receiving antennae can be constructed with relatively high quality factor Q, such that most of the resistive loss in the receiving circuit originates in the in vivo tissue sample. This is particularly important as the sensitivity of the NMR experiment requires that the receiving antenna favor the NMR response signal from a particular small excited volume of the sample, while being relatively insensitive to noise currents flowing through the total capture volume of the receiving coil.
It is also known that the radio-frequency (RF) fields generated by a simple loop or spiral surface coil are highly non-uniform. The surface coil reception sensitivity, which is essentially the inverse of the excitation field generated during sample irradiation, is likewise non-uniform. Hence, a relatively large RF antenna is required for transmission excitation of the sample to produce a more uniform irradiating RF field. A relatively small, but sensitive, surface receiving coil is utilized with the larger-diameter exciting surface coil.
Hitherto, the requirements for a relatively small-diameter receiving surface coil and a relatively large-diameter exciting surface coil has typically required that the NMR system antennae apparatus 10 (see FIG. 1) position the larger-radius R excitation antenna 11 in a first plane, e.g. in the Y-Z plane (for a three-dimensional Cartesian coordinate system having the NMR static imaging field B.sub.O directed in the Z direction), and position the receiving antenna 12, having a diameter r no greater than one-half the exciting antenna radius R, in a second plane, e.g. the X-Z plane, essentially orthogonal to the exciting transmitter first plane, e.g. the Y-Z plane. The essentially orthogonal placement of the exciting and receiving coils 11 and 12 is based upon several phenomena: the need to prevent currents (induced in the receiving coil during the presence of an irradiating RF magnetic field B.sub.x, e.g. in the X direction, for the illustrated transmitting coil in the Y-Z plane) from damaging the sensitive reception preamplifier, typically connected to receive coil terminals 12a and 12b to receive the induced reception signal voltage V.sub.r thereat; the need to prevent the currents induced in surface coil 12 from, in turn, producing an RF magnetic field B.sub.y which would have a component in the X direction if the receive coil 12 were not situated exactly in the X-Z plane and which would cancel out a portion of the excitation magnetic field B.sub.x ; and the need to avoid the electrical coupling of transmitting coil 11 to receiving coil 12 after the excitation of the sample. The currents induced in reception coil 12 can be prevented from damaging the receive coil preamplifier by utilizing resonant circuitry, as at terminals 12a and 12b, to isolate the subsequent preamplifier (not shown) during periods when a large magnitude of an excitation voltage V.sub.t is present at the terminals 11a and 11b of the transmitting antenna. However, the production of an induced RF magnetic field has hitherto only been reduced by the aforementioned essentially orthogonal placement of the two surface coils 11 and 12, and the art has not otherwise considered the problem of surface coil-to-surface coil coupling in the receive mode, which coupling causes criticality in the tuning adjustments of receiving coil 12 due to the relative orientation of coils 11 and 12 and can induce additional noise in the receiving antenna 12 caused by noise currents in the transmitting coil 11.
It is especially desirable, to facilitate placement of the antennae during in vivo imaging of a portion of the human anatomy, to have both the transmission excitation surface coil antenna 11 and the response signal receiving antenna 12 in a substantially planar configuration as, for example, described and claimed in application Ser. No. 641,540, filed on even date herewith, assigned to the assignee of the present application and incorporated herein in its entirety by reference. A highly desirable NMR imaging antenna has at least two surface coils, at least one of which is utilized for excitation signal transmission and at least one other one of which is utilized for response signal reception, but which are so decoupled as to be devoid of induced counter fields during excitation irradiation and to be devoid of damping and other deleterious effects during image signal reception.